1. Field of the Invention
This invention relates to an electric power steering unit to be used for a vehicle and, more particularly, it relates to a field of technology effectively applicable to electric power steering gears to be used for rack and pinion type steering systems.
2. Related Art Statement
In recent years, most vehicles are equipped with a so-called power steering gear, and various type power steering unit, such as a hydraulically or electrically operated type power steering unit has been designed to assist the steering power of the vehicle. In these electric power steering units, as one for applicable to rack and pinion type steering systems, a unit which the steering assisting power is obtained by an electric motor arranged coaxially with a rack-shaft, as Japanese Patent Application Laid-Open No. 8-98451, is known.
Such an electric power steering unit shown in FIG. 11, comprises an electric motor 52 arranged coaxially with a rack-shaft 51 so that the steering assisting power generated by the electric motor 52 is transmitted to the rack-shaft 51 by way of a ball screw mechanism 53. Then, the guiding wheels of the vehicle can be turned by utilizing both the manual steering power of the driver and the steering assisting power.
The rack-shaft 51 is linked to the guiding wheels (which hereinafter may simply be referred to as "the wheels") typically by way of tie rods or knuckle arms arranged at the respective opposite ends thereof and also linked to the steering column 54 that is coupled to the steering wheel (which hereinafter may be referred to as "the handle") by way of a rack and pinion gear so that it may be reciprocatively moved in the horizontal directions of FIG. 11 as the steering operation by the driver. The electric motor 52 has a cylindrical yoke 55 containing coaxially therein a cylindrical armature shaft 56 and a field device 57 and is fed with power from a power supply section 58. The field device 57 comprises magnets 59 arranged on the inner peripheral portion of the yoke 55 and an armature core 60 arranged on the outer peripheral portion of the armature shaft 56. The rotary power generated by the electric motor 52 is transmitted to the rack-shaft 51 and the power is converted into the reciprocatively movement by way of a ball screw mechanism 53 arranged at the left end of the armature shaft 56 in FIG. 11, so that steering power is assisted. Note that the armature shaft 56 is supported at a right side portion thereof by an angular bearing 65 held within housing 61.
With a power steering unit having the above described configuration, the rack-shaft 51 is supported at two points including the coupling section connecting itself to the steering wheel 54 and the ball-screw mechanism 53. Firstly, since the rack-shaft 51 is coupled at an end thereof to the steering wheel 54 by means of a gear engagement by a rack and pinion gear system, rigidity of this point is high and it provides a point of support for the rack-shaft 51. Additionally, a housing 61 holding the coupling section is rigidly secured to the vehicle main body so that the rack-shaft 51 is in effect supported by the vehicle main body. Secondly, rack-shaft 51 is supported at the opposite end thereof by a ball-screw mechanism 53 arranged at an end of the armature shaft 56. The ball-screw mechanism 53 comprises a nut section 62 and a screw section 63 that are tightly combined together with balls 64 interposed therebetween. The nut section 62 is press-fit into and caulked against the armature shaft 56 to provide another point of support for the rack-shaft 51.
The armature shaft 56 is rotatably held by the ball-screw mechanism 53 and a angular bearing 65. Thus, the ball-screw mechanism 53 provides a point of support for the armature shaft 56, while the housing 61 provides another point of support for the armature shaft 56 as the angular bearing 65 is tightly fitted to the housing 61.
The performance of a power steering unit of the type under consideration is typically measured in a manner as described below. If the electric motor 52 is installed after fitting the nut section 62 of the ball-screw mechanism 53 to the armature shaft 56 and subsequently the yoke 55 is placed in position, the entire unit is covered by outer shell elements so that the performance of the electric motor 52 can be determined only by way of the thrust of the rack-shaft 51 because it is no longer possible to directly measure the performance of the electric motor 52 itself. Therefore, for the purpose of the present invention, the electric motor 52 is measured for its performance before fitting the nut section 62 to the armature shaft 56. In other words, the unit is assembled and covered by the yoke 55 without the nut section 62. Next, the revolutions per minute and the torque of the electric motor 52 are measured by using the space where the nut section 62 is to be installed. After the operation of measuring the performance of the electric motor 52 is over, the yoke 55 is removed temporarily and the nut section 62 is placed in position, then the yoke 55 is put back again to complete the assembling operation.
The ball screw mechanism 53 has a well known constitution, which comprises a large number of balls 64 arranged between a nut section 62 and a screw section 63 thereof, the nut section 62 being press-fit into and caulked against the armature shaft 56. With this arrangement, the rotary power of the electric motor 53 is transmitted to the rack-shaft 51 by way of the nut section 62, the balls 64 and the screw section 63 to produce axial reciprocative power there, which is then used to assist the steering power.
When the guiding wheels of the motor vehicle is turned to an extreme, one of the tie rod stoppers (not shown) arranged at the both opposite ends of the rack-shaft 51 comes to abut the housing 61 and the yoke 55 at the end portion thereof to limit the turning motion of the wheels. As the stopper abuts these corresponding components, the rotary members of the armature shaft 56 revolving with the rack-shaft 51 are also stopped and the rack-shaft 51 is subjected to an impact due to the force of inertia of the rotary members. Then, the armature shaft 56 receives the reaction force from the ball-screw mechanism 53.
If, for example, one of the stoppers abuts the left end of the yoke 55 shown in FIG. 11, the rack-shaft 51 is subjected to force trying to move it further right in FIG. 11 due to the inertia of the armature shaft 56 and other related components. Then, the armature shaft 56 receives the reaction force of the rack-shaft 51 in the form of tensile force. If, on the other hand, the other stopper abuts the housing 61 at the right end in FIG. 11, the rack-shaft 51 is subjected to force trying to move it further left in FIG. 11 due to the inertia of the electric motor 52 and then the armature shaft 56 receives the reaction force of the rack-shaft 51 in the form of compressive force. Thus, with the power steering unit of FIG. 11, the armature shaft 56 is subjected not only to rotary force but also to tensile force and compressive force and hence the armature shaft 56 is solidly coupled to the nut section 62 in such a way that it seems as if the nut section 62 were wrapped by the armature shaft 56.
However, with such a conventional power steering unit, while the rack-shaft 51 apparently has two points of support, it in effect has three points of support because the armature shaft 56 linked to the rack-shaft 51 by way of the ball-screw mechanism 53 is also supported by the angular bearing 65 that is fitted to the housing 61, so that consequently the rack-shaft 51 can be more often than not subjected to friction due to jarring, twisting and misaligning of related components and that can give rise to problems, such as jarring, an unpleasant steering feeling and reluctant movement of the steering wheel in recovering. While, since very severe precision is required for each component and assembling to avoid these problems, there is such a problem that the manufacturing cost becomes high in terms of machining precision and product management.
Additionally, with a conventional power steering unit, the yoke 55 installed in position has to be temporarily removed to measure the performance of the electric motor 52 and then put back in position once again. In addition to the fact that this is a cumbersome procedure, there arises a problem that the performance of the electric motor 52 may behave differently when it is measured for performance and when it is completely assembled. In this case, the because the performance of the electric motor 52 is not tested at the complete assembling state and its performance is only estimated on the basis of the data obtained before re-assembling.
Still additionally, the yoke is already provided with unmagnetized magnets, which are magnetized after it is combined with an armature because a yoke having magnetized magnets can hardly assemble to armature. However, since the yoke 55 is removed to measure the performance of the electric motor 52 after magnetizing the magnets, the yoke 55 is still accompanied by the problem of removing. In the first place, it is not easy to pull out the magnetized yoke 55 from its position. Secondly, the yoke 55 attracts the armature vigorously and abut it due to its magnetic force to damage the surrounding elements.
With such a power steering unit, the armature shaft 56 has a complicated profile as shown in FIG. 11 and can be machined only with difficulties. Additionally, the nut section 62 is press-fit into and caulked against the armature shaft 56 in such a way that it seems as if the nut section 62 were wrapped by the armature shaft 56. This arrangement is entailed by an assembling problem.
Finally, since the armature shaft 56 is subjected not only to rotary force but also to tensile force and compressive force, it has to meet rigorous strength-related requirements particularly in terms of material, manufacturing process and machining accuracy despite that it has such a complicated profile to consequently give rise to a problem of high manufacturing cost.